1. Field of the Invention
Embodiments of the present invention relate generally to chromatic dispersion compensation in optical communication systems and, more particularly, to tunable dispersion compensators configured for continuous setpoint control.
2. Description of the Related Art
In optical communication systems with high data rates, such as 40 Gbs and faster, there is significant transmission penalty caused by chromatic dispersion in the optical media making up the system. Chromatic dispersion can seriously impact the propagation of light pulses in an optical system, because any light pulse has a finite spectral width, or bandwidth, and chromatic dispersion can therefore cause the different frequency components of a light pulse to propagate with different velocities. As a result, this variation in velocity causes the light pulses of an optical signal to broaden as they travel through the optical media. This phenomenon, known as “pulse spreading,” can cause increased bit error rates if left unchecked.
Fixed dispersion compensators are generally used in optical systems to perform bulk dispersion compensation, leaving only residual chromatic dispersion. Residual chromatic dispersion is an artifact of the imperfect match between the chromatic dispersion of an optical signal and a fixed dispersion compensator used to perform bulk dispersion compensation at a particular location in the optical system. Because the closely spaced light pulses in high-speed optical systems are generally more susceptible to pulse spreading, at high bit rates, e.g., 40 Gbps and faster, residual chromatic dispersion can create significant transmission penalties for such systems. Long-reach optical systems such as those used in submarine networks are also similarly susceptible at lower bit rates, e.g. 10 Gbps,
Tunable dispersion compensators (TDCs) are generally used throughout an optical communication system to compensate for residual chromatic dispersion, and can be precisely tuned to cancel the effects of chromatic dispersion at a specific location in the system. Chromatic dispersion is known to be the rate of change of the optical group delay response of a light pulse of an optical signal as a function of wavelength. Thus, one approach to compensating for chromatic dispersion involves passing the optical signal through a TDC that produces a rate of change of the optical group delay response with respect to wavelength opposite to that caused by the optical medium.
Problems arise, however, when the dispersion setpoint for a TDC is changed, which occurs periodically, such as when an optical channel is added to a node in an optical system or re-routed via a different optical link having different dispersion characteristics. Provisioning a link with a transponder that contains a TDC involves monitoring the eye open penalty (EOP) or bit error rate (BER) of the link and using the TDC to optimize EOP using a convergence algorithm. With a newly provisioned link, the dispersion setpoint for the TDC is also generally set to a new value. As the dispersion of the TDC is tuned abruptly from one setpoint value to another, the optical channel typically loses continuity, i.e., the optical channel passes through periods of unpredictable signal distortion due to uncontrolled dispersion of the TDC before arriving at the desired state.
Specifically, when the dispersion setpoint for an elaton-based TDC is changed, the TDC modifies the optical group delay response for each tunable etalon contained therein in order to provide the newly requested dispersion compensation value. If the TDC abruptly changes the optical group delay response of the etalons to the new setpoint, optical performance is generally not maintained, and for a finite time the optical channel may not comply with the performance specification of the optical system, a condition that is highly undesirable. This is because, as the dispersion is tuned from one setpoint to another, the optical channel may pass through time periods of unpredictable signal distortion due to uncontrolled TDC dispersion before arriving at the desired state. Alternatively, to ensure continuity of performance parameters of the optical channel at all times, the TDC can modify the spectral setpoint of each etalon via a series of small setpoint changes. At each small step change in dispersion setpoint, a PID (proportional-integral-derivative) controller minimizes overshoot and undershoot of all optical parameters of the optical channel, so that the TDC “settles” to a stable optical performance at each step. Such an approach prevents discontinuous optical performance of the optical channel, but is very time-consuming, e.g., on the order of several minutes. Thus, when a new dispersion setpoint is issued for a TDC, an optical communication system can suffer from either extended tuning time or periods of discontinuous optical performance.
Accordingly, there is a need in the art for a method of quickly adjusting a TDC from a first dispersion setpoint to a second dispersion setpoint that avoids discontinuous optical performance of an optical channel.